Over the past ten years digital radiation imaging has gradually been replacing conventional radiation imaging for certain applications. In conventional radiation imaging applications, the detecting or recording means is a photosensitive film or an analog device such as an Image Intensifier. Digital radiation imaging is performed by converting radiation impinging on the imaging device to an electronic signal inside a converting material and consequently digitizing such electronic signal.
Devices for performing digital radiation imaging currently exist, and typically fall into two classes: direct radiation detection and indirect radiation detection. In direct radiation detection devices, the impinging or incident radiation is converted locally into electrical charge which is then collected at collection contacts/detector pixels, and then communicated as electronic signals to readout circuits. The readout circuits perform various functions including digitization.
Direct radiation detection devices typically comprise a photo-conductor or detector substrate which converts the impinging radiation into electronic signals, and a readout substrate which receives, processes and reads out the electronic signals for imaging. There are different kinds of photo-conductor/detector substrate technologies and as well as different readout substrate technologies used in direct radiation detection devices. These include: SBBASIC (Semiconductor Bump-Bonded on ASIC), a-SGTFT (amorphous Semiconductor Grown on TFT), and a-SGASIC (amorphous Semiconductor Grown on ASIC). ASIC stands for Application Specific Integrated Circuit and TFT stands for Thin Film Transistor array.
SBBASIC-type imaging devices typically comprise at least two substantially coplanar semiconductor substrates: a crystalline semiconductor detector/photo-conductor substrate discretely bonded to a semiconductor readout substrate. Typically, the detector/photo-conductor substrate has a first major surface for receiving radiation impinging on the device, and a second opposite major surface on which is disposed a two dimensional array of detector pixels. Incoming radiation impinges on the first surface of the detector substrate and is absorbed in the thickness of the photo-conductor material. In response to the absorption of the radiation, electrical charges are generated in the photo-conductor material. For example, if the photo-conductor material is CdTe, 45 keV of impinging radiation energy may generate a charge of about 10,000 electrons, and similarly, 70 keV of radiation energy may generate a charge of 15,500 electrons, 100 keV of radiation energy may generate about 22,000 electrons, and 140 keV of radiation may generate about 31,000 electrons. Other photo-conductor materials may generate different levels of charge on the absorption of similar levels of impinging radiation, but in a similar manner.
Under the influence of an electrical field, the generated charges drift toward and are collected at the charge collectors (or charge collection electrodes) at the second surface of the detector substrate. Each charge collector contact defines a “detector pixel” on the detector substrate's second surface. Each detector pixel is conductively connected to a “pixel circuit input” on the adjacent surface of the readout substrate. In a SBBASIC-type imaging device, the connection between a detector pixel and a pixel circuit input is accomplished by a bump-bond. In photon/pulse counting SBBASICs, each pixel circuit input is an input to an ASIC pixel counting circuit processed onto the readout substrate. The ASIC pixel counting circuit can include a plurality of various circuits or features for amplifying, storing, digitizing, etc. the electrical charge signals from the detector substrate and count the photons absorbed or the charge pulses generated.
Photon or pulse counting imaging devices have stimulated considerable interest in both the scientific and commercial communities because they offer the potential for some significant advantages over other related technologies:                1. Because the charge pulse generated by each photon is processed individually it can provide information about the energy of the photon absorbed. Thus photons can be counted or discarded depending on their energy level. This in turn enhances the contrast resolution because photons of lower energy are typically scattered photons that, unless discarded, would deteriorate image quality.        2. Since electronic signals due to photon pulses are digitized and counted the device is less sensitive to background noise, detector leakage/dark current etc. Note, “dark current” is the background current flow in the device absent the presence of impinging radiation.        3. There is no need for outside digitization, because it is done “on chip,” which makes the imaging system simpler and potentially less expensive.Recognizing these advantages, the field has been motivated to develop photon/pulse counting digital imaging devices embodying them. U.S. Pat. Nos. 6,248,990 and 6,355,923 to Pyyhtia el al. are relatively exemplary of some of the latest efforts to embody the advantages of photon/pulse counting in digital imaging devices.        
FIGS. 1A and 1B generally illustrate an array of pixel cells 20 typical of the prior art in the field and as taught by the Pyyhtia '990 patent. The pixel cell 20 comprises a single detector pixel 36 in electrical communication with a single pixel counting circuit 31 on the readout semiconductor substrate 32. The charge collector electrode 38 of the detector pixel 36 is processed onto the pixel surface 40 of the detector semiconductor substrate 30. The charge collector electrode 38 is electrically connected to the pixel circuit input 33 of the pixel signal counting circuit 31 on the readout surface 42 of the readout semiconductor substrate 32 via a pixel contact (bump-bond) 35. The photo-conductor material 34 of the detector pixels 36 absorbs incoming radiation, and in response to the absorption generates electrical charges. The electrical charges are collected at the charge collector electrodes 38, and electrically communicated through the pixel contact bump-bonds 35 to the pixel circuit input 33 of the pixel signal counting circuit 31 on the readout semiconductor substrate 32. Also see Orava et al., U.S. Pat. No. 5,812,191 and Spartiotis et al., U.S. Pat. No. 5,952,646, which disclose alternative embodiments of an SBBASIC-type digital radiation imaging devices, FIG. 1B as an alternative illustration of the prior art embodied in FIG. 1A. Very recently, Pyyhtia et al., U.S. Pat. No. 6,355,923 disclosed developments in the art field as moving in the direction of having each detector pixel 36 being associated with one or more than one pixel signal counting circuit 31. Specifically as shown in FIG. 2, in the prior art device of the '923 patent, a detector pixel 36 can be in electrical communication with more than one pixel signal counting circuit 31.
The photon/pulse counting devices of the above cited prior art have a numerical relationship between the detector pixels and the pixel signal counting circuits such that the number of detector pixels is always equal to or less than the number of pixel cell counting circuits. Although this approach is seemingly straightforward and simple, it has certain potentially significant functional limitations relating to image quality or resolution due to: (1) hole trapping and (2) charge sharing between pixels.